Demolish the Speedway and use that plot for the ride. It’s the worst ride in the park now.If they were to add the ride to WDW, it doesn't seem like the area occupied by the circus tents itself is a large enough plot. Tokyo's ride building is rather enormous, from my rough measurements about 75,000-76,000 square feet (including the entrance and queue). I'm unsure if that entire space is occupied by the ride, or if there are other miscellaneous things that could be removed without any major impact. But until we see some blueprints, i'll err on the side of caution and assume that all of the space is necessary for the ride itself.
And if it does use that entire space, it'll stand alongside Disney's largest dark rides (at least ones that don't have multiple levels). To put this into perspective, the two large buildings at Disneyland's Fantasyland that contain their 5 classic dark rides (Pan, Snow, Toad, Alice and Pinocchio along with the included gift shops, restaurants and bathrooms) are less than 70,000 square feet combined.
Even being extremely generous (including the storage building behind the tents and hugging close against the service roads), the circus tents occupy at most about 66,000 square feet. If you include the circular courtyard nestled between Pete's sideshow and the Casey Jr Splash area, you can just barely get to 74,000 sqft. Mind you, either measurement would make the area insanely tight. Guests would be walking up against a tall and visible showbuilding. Without much room to properly hide it with decent scenery. Tokyo's is pulled farther back and hidden by a lot of extra surrounding scenery and forced perspective effects. It should also be mentioned that the shape of the building itself wouldn't fit into this space in the same way, the space is much more narrow than Tokyo's show building. So even if they were to squish the attraction into this space, the building and ride would need to undergo major re-engineering to fit. Something Disney is not particularly keen on these days (preferrng 1:1 clones right down to exact spacial dimension).
As Martin stated, the land nestled between the railroad and existing New Fantasyland is not an official expansion plot. Nevertheless, i'd still like to see it used. Besides the land itself needing work (and possible removal and relocation of what I assume is a retention pond up there), the service road would need to be rerouted and wrapped around whatever building they put there. Though I guess this is getting into armchair imagineering territory now.